SENSORY RECEPTORS IN THE ANTERIOR UVEA OF THE CATS EYE - AN IN-VITRO STUDY

Purpose. To identify electrophysiologically the functional types of se nsory fibers innervating the iris and the ciliary body of the cat's ey e. Methods. The uveal tract tract of cat's eye was excised and placed in a superfusion chamber. Recordings were made from single afferent un its of ciliary nerve branches responding to mechanical stimulation of the iridal surface, the ciliary body, and the choroid with a nylon fil ament or a glass rod. Chemical sensitivity was explored by applying ac etic acid, hypertonic NaCl, and bradykinin. Warm (60 degrees C) and co ld (4 degrees C) saline and a servocontrolled thermode were used for t hermal stimulation. Results. Thirty per cent of the studied population of sensory units (n = 95) were spontaneously active when the recordin g was started. Approximately 30% of the fibers conducted in the lowest range of the A-delta group; the remaining 70% were C fibers. Sustaine d mechanical stimulation of the receptive area elicited a tonic respon se in approximately 60% of the units, and a phasic response in the rem aining 40%. Exposure of the receptive field of mechanosensitive fibers to 600 mM NaCl evoked a long-lasting discharge in 50% of the units; a pplication of 1 to 10 mM acetic acid elicited a short discharge in 30% of the fibers, often followed by inactivation. Bradykinin (1 to 100 m u M) produced a long-lasting response in almost 50% of the units. Warm ing the receptive field recruited 20% of the explored units, whereas 1 7% were activated by low temperature. Conclusions. Two main functional types of sensory fibers innervating the iris and the ciliary body wer e distinguished: (1) mechanoreceptors, corresponding to afferent units sensitive only to mechanical stimuli were generally silent at rest, h ad relatively higher force thresholds, and discharged phasically in re sponse to long-lasting mechanical stimulation; (2) polymodal nocicepto rs, which were activated by mechanical as well as by chemical and/or t hermal stimuli, usually displayed spontaneous activity, had lower forc e thresholds, and fired tonically upon sustained mechanical stimulatio n.